Regent

A regent is a position of government in a country where there is a monarchy (like a king or queen). He (or she) is an "acting king", and does the job of a king where the real king is either too young, sick or cannot do what he is meant to for some other reason.

They may also be voted in if there are no monarchs in line to the throne, for instance, if the current king or queen does not have any relatives when s/he dies.